1. Field of Invention
In general terms, the invention relates to a product with an alignment mark thereon, a method of aligning a product and a method for manufacturing a device.
2. Related Art
A lithographic apparatus applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) of a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Substrates and patterning devices need to be very accurately aligned during exposure. It is known to provide optical alignment marks on the substrate and/or patterning device for this purpose. One known alignment mark has reflection properties that vary periodically as a function of position, so that the mark can be used as an optical grating. The period of such a grating may be sixteen micron for example, when light with a wavelength of about 0.6 micron is used to measure position. An optical system measures the position of the substrate and/or patterning device by forming an image of the diffracted light using selected orders of diffraction from this grating. In a simple alignment mark each period of the periodic variation is realized by one area of high reflection (e.g., metal) and one area of low reflection (e.g., oxide), for example one eight micron wide area of metal and one eight micron wide area of oxide in each period.
Two such alignment marks may be used, one wherein the periodic areas repeat in a first direction along the surface of the wafer (the X-axis) and one wherein the periodic areas repeat in a second direction along the surface of the wafer (the Y-axis). Moreover, typically alignment measurements are performed with light of two wavelengths.
However, an alignment mark with periods made up of two homogeneous areas tends to make use of homogeneous areas of sizes that are much larger than that of tracks in modern integrated circuits. In response, it is possible to construct the different areas in each period of the alignment mark from finer lines. From an article titled “Advances in Process overlay-Alignment Solutions for Future Technology Nodes”, in Metrology, Inspection and Process Control for Microlithography XXI (Chas N. Archie editor) Proc SPIE Vol 6518 by Henry Megens et al. an alignment mark is known wherein the periodic structure comprises alternately a first area with fine conductive tracks directed along a first direction and second areas with fine conductive tracks directed along a second direction perpendicular to the first direction. The distance between these conductive tracks is made smaller than the wavelength of the light that is used to measure the position of the alignment mark.
Such line patterns results in a wired grid polarization effect, whereby polarization components of the light with electric field components parallel and perpendicular to the tracks are mainly reflected and transmitted respectively. Because the direction of the tracks alternates in the alignment mark, such an alignment mark provides an offset between the periodically varying reflection properties for the respective polarization components. As a pattern of variation of the direction gives rise to diffraction: the pattern of directions acts as a diffraction grating that produces different orders of diffraction at different angles. Light from these orders of diffraction is used to measure the position of the alignment mark.
Unfortunately, this type of alignment mark is incompatible with the use of diffractive imaging techniques that use pairs of orders of diffraction. An example of such imaging technique is the dipolar illumination method. In the dipolar illumination method light falls on the mask at an oblique angle of incidence with respect to the optical axis of the illumination system. Light with an oblique angle of incidence will generate transmitted diffraction orders that have large diffraction angles. The projection system may be configured to capture only the zero and the first transmitted diffraction orders and projects them on the resist layer constructing an image of the grating. If the grating lines are perpendicular to the plane of incidence that contains the optical axis and the wave-vector of the incident beam, a sharp image of the grating will be formed on the resist layer. But if the gratings lines are parallel to this plane of incidence a poor resolved image of the grating will be formed on the resist layer. The poor image results in a grating with different parameters or no grating at all. If the alignment mark period consists of sub-wavelength lines that are perpendicular in directions, lines in one direction will be well resolved and the lines in the perpendicular direction will not be resolved under dipolar or polarized illumination. When alignment marks in the X and Y direction are used that are the same except for a rotation over ninety degrees, the same polarization component cannot be used to measure the position of both X and Y marks.